(1) Field of the Invention
This invention relates to an inkjet printing apparatus for printing on a printing medium by discharging ink droplets thereto, and a flushing method therefor. More particularly, the invention relates to a flushing technique for discharging ink droplets in order to prevent defective discharge due to ink drying.
(2) Description of the Related Art
A known apparatus of this type includes an inkjet head for discharging ink droplets, and a controller for causing the ink droplets to be discharged from the inkjet head and controlling flushing (see Japanese Unexamined Patent Publication No. 2009-90533, for example).
The inkjet printing apparatus having the above construction further includes caps for covering discharge portions of the inkjet head in order to prevent drying of the ink droplets. When printing with this apparatus, these caps are first removed, and then transport of printing paper is started. The printing paper is accelerated toward a transport speed specified in printing conditions. After the transport speed reaches a printing speed and becomes substantially constant, printing is performed by discharging the ink droplets from the inkjet head to job areas of the printing paper.
While printing is started as described above, the controller carries out line flushing in advance to avoid missing nozzles due to defective discharge at the printing time. The following two techniques are typical examples used in the line flushing.
The first technique does not start printing immediately after the paper transport speed reaches the printing speed. Instead, it carries out line flushing (hereinafter called advance line flushing) for a pre-printing area, in which ink droplets are discharged from a plurality of discharge portions present in a row among a plurality of discharge portions of the inkjet head arranged perpendicular to the transport direction of the printing medium. Then, after start of the printing, it carries out star flushing (also called discrete flushing) in which the ink droplets are discharged discretely from each discharge portion of the inkjet head.
The second technique also carries out advance line flushing instead of starting printing immediately after the paper transport speed reaches the printing speed. After start of the printing and before printing pages to constitute a printed product, it carries out advance line flushing for a part corresponding to an area outside the pages.
However, the conventional examples with such constructions have the following problems.
With the conventional apparatus, whether the first technique or the second technique is employed, when a “followup printing” is carried out after printing on printing paper beforehand, the advance line flushing may be done to overlap the portion where printing has already been performed. Then, the ink becomes superfluous to impair drying performance, which may result in contamination of the interior of the apparatus caused by transfer through the rollers acting to transport the printing paper. The overlapping of the advance flushing may be avoided by inserting a flushing page for a subsequent advance line flushing at the time of followup printing. But this poses a problem of consuming extra printing paper.
Further, a post-processing machine such as for cutting printed paper, for example, is used to carry out a cutting process which uses page marks printed on upstream sides of page portions. At this time, the advance line flushing carried out before printing could be detected in error as page marks. Its prevention involves a problem of taking extra time and effort such as masking of the areas of advance line flushing.